1. Field of the Invention
The present invention relates to a key telephone system, key telephone, interface unit, and transmission scheme determination method and, more particularly, to data transmission between key telephones and a main unit, which realizes compatibility between a conventional key telephone system and a new system.
2. Description of the Related Art
A key telephone system is widely used in an office or the like, in which a main unit including a telephone interface unit accommodating a plurality of key telephones is connected to a public switched telephone network or an office line such as a leased line. Data transmission between each key telephone and the telephone interface unit is performed by a scheme unique to the system. The transmission scheme mainly depends on the transmission rate. New key telephones and main unit (telephone interface unit, in particular) can be separately bought to replace the old ones. If, however, only the telephone interface unit is replaced with a new one for a new system, the old key telephones for the old system cannot cope with the transmission rate required by the telephone interface unit for the new system, wasting the resources of the old system.
According to this conventional key telephone system, new key telephones and telephone interface unit cannot be separately bought to replace the old ones, and the overall system including many key telephones must be replaced with a new one. This imposes a heavy economical load on the user.